Neutron Stars in Binary Systems G. Borner, Munich (Max-Planck Institute for Physics and Astrophysics) The discovery of a 58 keV line fea­ sources, and sources showing irre­ already been established that they are ture in the spectrum of the X-ray gular fluctuations (as e.g. Cyg X-1 on members of a binary system (several source HerX-1 by J. TrÜmper and his a timescale of milliseconds) appear are listed in Table 1), i.e. they are in colleagues from Tubingen & Garching less exotic than the flare-like objects, orbit around an optical companion. (Germany) in 1976 '), as reported in which appear, reach a peak lumino­ As long as there is no evidence to Europhysics News, 8 (1977) 5, is the sity, and then fade away as if they the contrary, we may live happily with latest exciting news from the rapidly had never been. But most peculiar are the assumption that all other galactic expanding field of X-ray astronomy. the burst-like sources, which emit X-ray sources are also binary systems Cosmic X-rays are completely absor­ their X-rays in short term bursts of a with mass flow, and we may wait bed by the earth's atmosphere, and few seconds duration, with no appa­ confidently for further candidates to therefore the observing instruments rent periodicity involved. The theorists Table 1. have to be placed in the outer layers do not hesitate, however, to invoke Most of the energy is liberated or outside the atmosphere. Up to equally exotic images, when they try deep inside the gravitational well, i.e. 1970, balloon and rocket flights had to explain the observations. Although on the stellar surface, if the compact led to the discovery of about 30 X-ray we are far from understanding these object is a neutron star, or within a sources. But then, in December 1970, X-ray stars completely, we have lear­ few Schwarzschild radii, if the object NASA launched the Uhuru satellite ned quite a bit during the past years. is a black hole. In both cases, for 1 which was devoted entirely to X-ray Some general, fairly obvious, but not solar mass objects, distances are astronomy. This was the big step necessarily compelling conclusions $$ 106 cm. A proton in free fall on to forward, because within less than 2 (there is always a more complicated a 1 Mo object would have a kinetic years, Uhuru had found about 100 fairy tale with the same outcome) can energy of 100 MeV at such a dis­ galactic and about 50 extragalactic be drawn immediately from the data : tance. If one thermally radiates away X-ray sources. About 90 % of these The rapid variability on timescales the energy in X-rays of a few keV, i.e. galactic sources apparently constitu­ of seconds or milliseconds (cf. Table at temperatures of 7 $$ 107 to 3x107 K, ted a new class of stellar objects 1) suggests that a compact object from areas with such typical dimen­ with an X-ray luminosity exceeding (white dwarf, neutron star, black hole) sions, one finds luminosities of 1036 - the sun's total luminosity by a factor is involved. The energy source for the 1038 erg/s. 5 of 10 . This was substantiated by X-rays comes from gas which accre­ This is in good agreement with the satellite observations in later years. tes in the deep gravitational well of observations. Talking only of black The new satellites also revealed the compact object, flowing over from holes and neutron stars is, of course, new types of galactic X-ray stars, so a companion star, which is not com­ unfair to the white dwarfs, which can that now one knows of a large variety pact. This view is supported by the also satisfy the constraints imposed of types, of which regular pulsating fact that for 10 X-ray sources, it has by the data. But accretion on white Table 1 Source Short term variability Long term variability Optical candidate Distance Luminosity (2-10 keV) (kpc) (1036 erg) HerX-1 1.24 s pulsations X-ray eclipses (1.7d) HZ Her ; 2-6 1 -10 (6 kpc) (3U1653 + 35) 35d modulation 1.7d binary long-term on-off Cen X-3 4.84 pulsations X-ray eclipses (2.1 d) double ellipsoidal 6-9 10-30 (9) (3U1118-60) extended lows light variation Ob star Cyg X-1 Irreg. var. in times slow transition HDE 226868 2.5 3-10 var (3U1956 + 35) of milliseconds between 2 distinct 5.6d binary spectral states Vela X-1 283 s pulsations flares, eclipse HD 77581 2 10 (3U0900-40) (282.8913 ± 0.0004) (8.95d) SMCX-1 0.71 s pulsation X-ray eclipses, 3.9d SK 160 60 30 - 300 (3U0115-73) extended lows 13 mag. ScoX-1 Irreg. var. min slow (~ 10 min-h) ScoX-1, blue 1 -2 10 (3U1617-15) flares 0.787d period A0620-00 none appears in outbursts optical 1 100 lasting $$ 1 year Identification (1967 & 1975) no period determined tures of the X-ray source (in Fig. 1, therefore that HerX-1 is a neutron star. a schematic drawing of the gas flow It also establishes that neutron stars is presented). Table 1 (ref. 2) con­ really have the large magnetic fields tains several regularly pulsating X-ray expected for them from theoretical sources, with pulse periods in the considerations of stellar collapse, and range of 1.24 s (HerX-1) - 283 s (Vela from the slowing down of the pulsars X-1). The time-keeping mechanism is by the emission of electromagnetic probably rotation of the compact ob­ dipole radiation. Meanwhile a second ject, which means rotation of a neu­ experiment by Trümper ef al. has tron star at least for the fast rotators. confirmed the results of the first mea­ The observed pulses must be produ­ surements, and other observers have ced by an asymmetry in the infall also obtained evidence for a line fea­ of matter, and the obvious way to ture in the spectrum of HerX-1. achieve this is to have inclined to the The large value of 5. 108 T also in­ axis of rotation, a stellar magnetic dicates that the line emission is a field, which guides the matter down, quantum effect. In strong magnetic such that only certain hot spots on fields, the motion of the electron per­ the surface accrete. As these hot pendicular to the field is quantized, spots move through our field of vision, similar to the quantization of the elec­ we see X-ray pulses. tron orbits in an atom. The energy difference between two adjacent or­ Fig. 1. — Schematic drawing of accretion bits or levels is $$ωB_ where ωB = from a stellar wind and Roche Lobe Measurement of the Magnetic Field overflow with disk accretion. of a Neutron Star B B What information do we gain from eBlmc is the classical gyration fre­ dwarfs produces keV X-rays only, if observations on the physics of the quency. A spin reversal of the elec­ the infall is radial ; the observed accretion process close to a neutron trons has the same energy difference period changes are very difficult to star ? The spectrum of the radiation 2π h ωB. So there are two excited explain, and strong magnetic fields, emerges from a complicated and in­ states with the same energy diffe­ as measured by Trümper 1), cannot tricate interaction of many distinct rence with respect to a ground state : be supported by white dwarfs. With features, such as hot plasmas accele­ a higher orbit quantum number, or a accretion on to neutron stars or black rated by a strong gravitational field, different spin direction of the electron holes, about 10% of the rest mass guided by a rotating magnetic field, on the same orbit. Transitions into the energy of the infalling gas is conver­ finally hitting a neutron star surface ground state from the first excited ted into X-rays. Thus the accretion of very high density. There are so state in this system of Landau levels rates do not have to be very large : many parameters unknown, that a produce the observed line. The emis­ 8 10 sion of higher harmonics should also 10- Mo/a to 10- Mo/a is sufficient quantitative picture of the accretion to give the powers observed. process is not available, one only be possible. There is, indeed, an indication of an enhancement in the These or larger mass-transfer rates knows that it must be quite compli­ observed spectrum around 110 keV, can easily occur in close binary sys­ cated. where one would expect the second tems. Three different mechanisms There are great hopes, however, for harmonic to occur. may be important : an improvement of the theoretical i) A massive companion expanding situation, due to the measurement by Fig. 2. — Measurement of J. Trümper et across the critical equipotential the group of J. Trümper. They have al1) showing the spectrum of the HerX-1 surface between the two stars observed the regularly pulsating pulses and of OSO-8 (Aug. 75) showing the total spectrum. — Roche lobe overflow — could source HerX-1 for about 4 hours in a transfer mass at a rate of 10-6 balloon flight from Palestine/Texas on 3 Mo/a to 10- Mo/a. May 3, 1976. The spectrum for the ii) A stellar wind, I.e. an atmosphere pulsed flux they have obtained is streaming radially outwards, as shown in Fig. 2. There is strong evi­ expected from Ob or Be stars, can dence of a spectral line feature at 58 ± have a magnitude of 10-6 Mo/a.